The spinal column is a highly complex system of bones and connective tissues that provide support for the body and protect the delicate spinal cord and nerves. The spinal column includes a series of vertebral bodies stacked atop one another, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord is located behind the vertebral bodies. The spine has a natural curvature (i.e., lordosis in the lumbar and cervical regions and kyphosis in the thoracic region) such that the endplates of the upper and lower vertebrae are inclined towards one another.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), excess kyphosis (abnormal forward curvature of the spine), excess lordosis (abnormal backward curvature of the spine), spondylolisthesis (forward displacement of one vertebra over another), and other disorders caused by abnormalities, disease, or trauma (such as ruptured or slipped discs, degenerative disc disease, fractured vertebrae, and the like). Patients that suffer from such conditions often experience extreme and debilitating pain, as well as diminished nerve function.
A noteworthy trend in the medical community is the move away from performing surgery via traditional “open” techniques in favor of so-called “minimally invasive” or “minimal access” techniques. Open surgical techniques are generally undesirable in that they typically require large incisions with high amounts of tissue displacement to gain access to the surgical target site, which produces concomitantly high amounts of pain, lengthened hospitalization (increasing health care costs), and high morbidity in the patient population. Less-invasive surgical techniques (including minimal access and minimally invasive techniques are gaining favor due to the fact that they involve accessing the surgical target site via incisions of substantially smaller size with greatly reduced tissue displacement requirements. This, in turn, reduces the pain, morbidity, and cost associated with such procedures.
One disadvantage to performing minimally invasive surgery is the increased reliance on radiographic imaging to “see” the spinal target site, instruments, and implants during the surgery. While this increased exposure is generally negligible for the patient, over time and over multiple procedures on different patients, this increased exposure adds up for the surgeon and other operating room personnel. Systems and methods have been developed to reduce reliance on radiographic imaging during spine surgery. Once such system and method involves three-dimensional (3D) navigation systems that use positional tracking systems to track the position of implants and instruments relative to the spinal target site and present the surgeon with a representative image of the instrument superimposed on an image of the target site to indicate the position of the implant or instrument relative to the anatomical structures depicted in the image. (e.g. spinal target site). However, these systems have the disadvantages of generally requiring that reference markers be somehow fixed to the patient (e.g. anchoring a pin or other instrument into the patient's spine, thus causing additional trauma to the patient's anatomy), requiring input of pre-operative CT images into the system before or during the procedure, and/or requiring large, specialized, and expensive equipment that may not be available in certain instances or operating rooms. Furthermore, even though 3D navigation provides spatial information regarding a target surgical site, instruments, and implants during surgery, it does not provide neurophysiologic information regarding the nerves lying near and around the operative corridor.
A need exists for systems and methods that provide both information regarding the target spine site, surgical implants, instruments, and nerves surrounding the operative corridor during minimally-invasive spine surgeries. The systems and methods described herein are directed to addressing the challenges described above, and others, associated with various minimally-invasive spine procedures.